Minerals or concentrates containing tantalum and niobium are conventionally decomposed with HF or mixtures of HF/H.sub.2 SO.sub.4. The tantalum/niobium heptafluoro complexes formed may be purified by solvent extraction and separated (G. L. Miller, "Tantalum and Niobium", [Butterworths] London 1959).
The heptafluoro complexes obtained from solvent extraction are stripped into the aqueous phase and precipitated from this phase with ammonia. The tantalum/niobium hydroxides are then obtained by filtration and washing of the filter cake with dilute ammonia solutions. This process is described in U.S. Pat. No. 3,112,991 and in German Application 3,428,788.
When the fluoride ions cannot be thoroughly washed out of the filter cake, considerable problems of corrosion occur in the apparatus during the subsequent calcination to convert the products into low fluoride oxides. Under these conditions, exhaust gas purification of the roasting gases also becomes difficult.
Another disadvantage of this process is that it requires the use of a very large amount of ammonia per kg of oxide, which must finally be discharged unused into the effluent, which in turn must then be subjected to an extensive after-treatment to recover the ammonia.
It is therefore an object of the present invention to develop a process which would
a) enable the fluoride content to be reduced to a value below 0.5% by weight, preferably under 0.3%, and
b) would reduce the ammonia consumption and hence would reduce the amount of ammonia/ammonium salt solutions to be disposed of.